1. Field of the Invention
The present invention relates to an ultra-thin copper foil with a carrier and a method of production of an ultra-thin copper foil with a carrier, more particularly relates to an ultra-thin copper foil with a carrier suitable for a printed circuit board for high density ultrafine interconnect (fine pattern) applications.
2. Description of the Related Art
A printed circuit board is produced as follows: First, an electrically insulating board comprised of a glass-epoxy resin, a glass polyimide resin, etc. is covered on its surface by a thin copper foil for formation of surface patterns, then the two are heated and pressed to produce a copper-clad laminate.
Next, this copper-clad laminate is successively formed with through holes and plated at the through holes, then the copper foil at the surface of the copper-clad laminate is etched to form interconnect patterns of the desired line widths and line pitches. Finally, a solder resist is formed and other finishing work is performed.
The copper foil used for the copper-clad laminate is roughened at its surface at the side to be hot bonded to the board, exhibits an anchoring effect on the board by the roughened surface, and thereby raises the bond strength between the board and copper foil to secure reliability in the printed circuit board. Further, recently, a printed circuit board, in particular a build-up circuit board has been made by using a resin coated copper foil whose roughened surface is covered in advance by a bonding resin such as an epoxy resin which is made to an insulating resin layer in the semicured state (B stage) as the copper foil for forming the surface circuits, when the insulating resin layer side is hot bonded to a board. A “build-up circuit board” is a type of multilayer circuit board obtained by forming an insulating layer and conductor patterns one by one in turn on an insulating board, plating holes (vias) formed by a laser method or photolithography, and stacking the interconnect layers while connecting the layers conductively.
This circuit board can handle the increasingly higher densities of various electronic parts. By making the vias increasingly fine, the interconnect patterns can also be made higher in density. Therefore, there is a demand for a printed circuit board with interconnect patterns enabling interconnects of fine line widths and line pitches, that is, fine patterns. For example, in the case of printed circuit boards used for semiconductor packages, a printed circuit board having high density ultrafine interconnects of line widths and line pitches of around 30 μm has been demanded.
If a thick copper foil as the copper foil for forming such a fine printed circuit board is used, the etching time until reaching the surface of the board becomes longer. As a result, the verticality of the side walls of the interconnect patterns formed is ruined. In the case of interconnect patterns with narrow line widths of interconnects in the interconnect patterns formed, sometimes this leads to disconnects. Therefore, as the copper foil used for fine pattern applications, copper foil having a thickness of less than 9 μm, in particular less than 5 μm due to the increase in the number of circuits due to the increase in the volume of information, is being used.
However, a thin copper foil (ultra-thin copper foil) is low in mechanical strength, easily wrinkles, is creased, and sometimes tears when producing a printed circuit board, so as the ultra-thin copper foil used for fine pattern applications, an ultra-thin copper foil with a carrier obtained by directly electrodepositing an ultra-thin copper foil layer on one surface of a carrier copper foil through a peeling layer is being used.
The ultra-thin copper foil with a carrier, as shown in FIG. 4, is formed with a foil 1 serving as the carrier (hereinafter called the “carrier foil”) and a peeling layer 2 and electroplated copper layer 3 successively formed on one surface of the same. The surface of the outermost layer of the electroplated copper layer 3 is roughened. Further, the roughened surface is superposed with a glass epoxy board, the two are hot bonded, then the carrier foil 1 is peeled off via the peeling layer 2 to expose the bonding side of the electroplated copper layer 3 with the peeling layer 2, and the electroplated copper layer 3 is formed with predetermined interconnect patterns.
The carrier foil 1 functions as a reinforcing material (carrier) backing up the electroplated copper layer 3 until bonding the thin electroplated copper layer 3 with the board. Further, the peeling layer 2 is a layer for improving the peeling when separating the electroplated copper layer 3 and the carrier foil 1 and is removed together with the carrier foil 1 when peeling off the carrier foil 1, so it is possible to cleanly and easily peel off the carrier foil 1. On the other hand, the electroplated copper layer 3 bonded with the glass epoxy board is successively formed with through holes and plated at the through holes, then the copper foil at the surface of the copper-clad laminate is etched to form interconnect patterns with the desired line widths and line pitches. Finally, a solder resist is formed and other finishing work performed.
In this way, the copper foil with a carrier can be used to produce a build-up circuit board since the thickness of the electroplating layer 3 can be made an ultra-thin one of for example 9 μm or less, fine patterns can be formed, and the handling ability is superior.
As the copper foil with a carrier, a composite foil providing a carrier foil with a chromate thin peeling layer, providing the peeling layer with a copper layer by an alkaline copper pyrophosphate bath, and providing this with a further copper layer has been proposed (for example, see Japanese Examined Patent Publication (Kokoku) No. 61-34385).
When applying the composite foil to a heat resistant glass epoxy resin laminate such as the FR-4 grade, the hot bonding temperature is around 170° C., so the copper foil and carrier foil can be peeled apart, but the peel strength depends on the surface roughness of the carrier copper foil and the peel strength lacks stability. Further, when a heat resistant resin, in particular a polyimide resin, is used as a board, the working temperature becomes a high temperature of 300° C. or more both in the case of the casting method and hot bonding method, so diffusion of copper due to the temperature becomes great, the surface roughness of the carrier copper foil further affects the peel strength, the stability of the peel strength of the carrier foil and ultra-thin copper foil is lost, and the variation in the peel strength also becomes large. Further, since it is difficult to uniformly plate an ultra-thin copper foil on a peeling layer, the number of pinholes present in the ultra-thin copper foil becomes greater (for example, see Comparative Examples 5 to 6 of Japanese Examined Patent Publication (Kokoku) No. 08-18401).